Conventionally, a contra-rotating propeller has been known in which an outer shaft having a front propeller and an inner shaft fitted in the outer shaft and having a rear propeller are caused to rotate in mutually opposite directions in order to effectively utilize propeller propulsion energy. Recently, the contra-rotating propeller has been employed in order to enhance the propulsion efficiency of a ship propulsion system.
For example, FIG. 28 is a partial broken side view showing a contra-rotating propeller according to the prior art, and FIG. 29 is a sectional view taken along the line E--E shown in FIG. 28. A contra-rotating propeller 100 includes a hollow outer shaft 102 having a front propeller 101, an inner shaft 104 provided in the outer shaft 102 and having a rear propeller 103, and a main engine 105 for causing the outer shaft 102 and the inner shaft 104 to rotate in mutually opposite directions. The outer shaft 102 has a cylindrical shape, and is rotatably provided on a stern portion 106 of a hull Y through an outer shaft bearing 107 and an outer shaft seal 108. The inner shaft 104 is provided inside the outer shaft 102 through a contra-rotating bearing 109 and an inner shaft seal 110 so as to rotate in the opposite direction. The hull Y is provided with a lubrication oil supply system 111 for supplying lubrication oil to the outer shaft 102, the inner shaft 104, the outer shaft bearing 107 and the contra-rotating bearing 109. A rudder horn 112 and a rudder plate 113 are opposed to the front propeller 101 and the rear propeller 103.
In the contra-rotating propeller 100 having such a structure, an ordinary journal bearing mechanism can be employed as the outer shaft bearing 107 provided between the outer shaft 102 and the stern portion 106. In the contra-rotating bearing 109 which supports the inner shaft 104 that rotates in the outer shaft 102 in the opposite direction, particularly as shown in FIG. 29, however, in the case where the inner shaft 104 rotates clockwise and the contra-rotating bearing 109 such as a plain bearing which is fixed to the outer shaft 102 and the inner peripheral face thereof rotates counterclockwise, there is a possibility that lubrication oil cannot form an oil film between the outer peripheral face of the inner shaft 104 and the inner peripheral face of the contra-rotating bearing 109 if the outer shaft 102 and the inner shaft 104 rotate at almost equal speeds. For this reason, it becomes hard for the shafts 102 and 104 to perform hydrodynamic lubrication due to the oil film formation by the lubrication oil supplied from the lubrication oil supply system 111 when the inner shaft 104 and the outer shaft 102 rotate in mutually opposite directions at almost equal speeds as shown in a sectional view of FIG. 30 in which the contra-rotating bearing 109 is provided on the inner peripheral face of the outer shaft. Thus, there is a possibility that the ordinary bearing cannot display the bearing function.
For this reason, in some bearings, a floating bush 114 is provided between the outer shaft 102 and the inner shaft 104 as shown in a sectional view of FIG. 31. And, the floating bush 114 is almost kept stationary between both shafts 102 and 104 so that an oil film is formed between the floating bush 114 and each of the inner shaft 104 and outer shaft 102 to form a contra-rotating bearing. In case of a hydrodynamic bearing using the floating bush 114, however, an oil film which can display the bearing function can be formed during ordinary operation, while the oil film is formed with difficulty during low-speed operation when entering and leaving a port so that metallic contact is sometimes caused on the bearing surface. Furthermore, the inner shaft 104 is bent easily by a cantilever support of the rear propeller 103 so that local contact may be caused on the rear end of the bearing.
In some bearings, a tapered land (not shown) is formed on the inner face of the contra-rotating bearing 109 which supports the inner shaft 104, and a dynamic pressure is generated by a tapered portion of the tapered land to form an oil film so that the inner shaft 104 is lifted up. Also in the case where the tapered land is provided, however, a load carrying capacity generated by the dynamic pressure is small when causing the main engine 105 to start or rotate at a low speed. Consequently, the oil film becomes thinner. Thus, there is a possibility that the inner shaft 104 and the contra-rotating bearing 109 metallically comes in contact with each other on the bearing surface so that the contra-rotating bearing 109 is seized.
As the prior art to solve the above-mentioned problems, "STERN TUBE BEARING SYSTEM OF CONTRA-ROTATING PROPELLER" (Japanese Examined Patent Publication No. 5-45479) which is based on a hydrostatic bearing as shown in a sectional view showing main parts of FIG. 32 has been disclosed, wherein an oil feed hole 116 is formed radially from a concentric hole 115 of the inner shaft 104, and a screw 117 having a small hole for orifice formation or capillary tube restriction is fitted in the oil feed hole 116 to eject high-pressure oil from the oil feed hole 116 toward the contra-rotating bearing surface between the inner shaft 104 and the outer shaft 102 or the contra-rotating bearing 109 so that a load carrying capacity is generated by a static pressure to lift up the inner shaft 104, thereby preventing the local contact of the inner shaft 104. In addition, a contra-rotating bearing has been disclosed in which a plurality of oil feed holes 116 which are radially provided and connected with both of the outer peripheral face of the inner shaft within a range L of the bearing surface of the inner shaft 104 and the concentric hole 115 provided on the core of the inner shaft 104, and the oil feed holes 116 are arranged in a plurality of lines in the longitudinal direction of the inner shaft as shown in sectional views of FIGS. 33 (a) and (b).
In case of above-mentioned contra-rotating bearing, however, the contra-rotating bearing 109 of the outer shaft 102 is circular bearing. For this reason, when the inner shaft 104 and the outer shaft 102 rotate in mutually opposite directions at equal speeds as described above, the circular bearing does not generate a load carrying capacity by the dynamic pressure of the lubrication oil theoretically. Consequently, in case of a high ratio of number of revolutions in which the inner shaft 104 and the outer shaft 102 (contrarotating bearing 109) rotate in mutually opposite directions at almost equal speeds, there is a possibility that an oil film is not formed but seizure is easily caused when the static pressure oil supply from the radial oil feed hole 116 is not performed due to blackout or the failure of an oil feeding pump of the lubrication oil supply system 111.
In case of the high ratio of number of revolutions, the load carrying capacity generated by the dynamic pressure is insufficient as described above. For this reason, it is necessary to supply the lubrication oil from the radial oil feed hole 116 with a comparatively high static pressure. Consequently, the size of the lubrication oil supply system 111 becomes larger. Furthermore, a lot of oil feed holes 116 are provided on the inner shaft 104 to form the static pressure bearing. For this reason, the strength of the inner shaft 104 is sometimes lowered. In such a case, it is necessary to increase an inner shaft diameter in order to maintain the strength of the inner shaft. Consequently, a weight is increased and the size of a propulsion unit becomes larger.
The present invention has been made in order to provide a contra-rotating bearing device for a contra-rotating propeller wherein a sufficient oil film is formed on the bearing surface of a contra-rotating bearing which supports an inner shaft in the contra-rotating propeller so that an excellent bearing function can be displayed.
It is an object of the present invention to provide a contra-rotating bearing device for a contra-rotating propeller which can lubricate the contra-rotating bearing by using a small number of oil feed holes without reducing the strength of the inner shaft.
It is another object of the present invention to provide a contra-rotating bearing device for a contra-rotating propeller which can supply lubrication oil having a pressure which is necessary within the range of the bearing surface of the inner shaft by simple working.
It is yet another object of the present invention to provide a contra-rotating bearing device for a contra-rotating propeller which can particularly give a load carrying capacity to the contra-rotating bearing during uniform contra-rotation in which an inner shaft and an outer shaft rotate in mutually opposite directions at almost equal speeds, or in the state close thereto.
It is a further object of the present invention to provide a contra-rotating bearing device for a contra-rotating propeller to surely keep a load carrying capacity by supporting the inner shaft with only the load carrying capacity generated by the dynamic pressure of the lubrication oil in the high rotation area of an engine and by increasing an oil feeding pressure to add a static pressure in a low rotation area in which it is hard to generate the sufficient load carrying capacity by only the dynamic pressure.
It is a further object of the present invention to provide a contra-rotating bearing device for a contra-rotating propeller which can generate the sufficient load carrying capacity with a low oil feeding pressure by using the non-circular shape of the contra-rotating bearing.